This invention relates in general to instruments for scanning samples or specimens and in particular to a system for scanning samples or specimens with improved characteristics.
Profiling instruments were first developed for the purpose of characterizing surfaces in terms of roughness, waviness and form. In recent years, they have been refined for precise metrology in the measurement and production control of semiconductor devices. Profiling instruments are also used outside the semiconductor industry, for example, for scanning and sensing optical disks, flat panel displays, and other devices.
Stylus profilometers for use in the above-mentioned applications have been available from Tencor Instruments of Mountain view, Calif., and other manufacturers. In a conventional stylus profilometer, a sample is placed on an X-Y positioning stage, where the surface of the sample to be measured or sensed defines the X-Y plane. The stylus profilometer includes a stylus tip brought to a position relative to the sample to sense certain interactions between the stylus tip and the surface of the sample. The stylus and stylus tip are attached to an elevator which moves in a Z direction that is perpendicular to the X-Y plane. The sensor does not move in X or Y directions (i.e., directions in the plane parallel to the surface of the sample). The interactions between the stylus tip and the sample are measured by the sensor. In a data acquisition sequence, the X-Y stage moves the sample in a controlled manner under the stylus tip while the sensor senses variations of sample-stylus tip interactions across the sample surface as the sensor scans the sample surface. Thus during data acquisition using the sensor, the X-Y stage is moving the sample in a controlled manner.
The Alpha-step.RTM. is another type of stylus profilometer available from Tencor Instruments of Mountain view, Calif. The Alphastep scans a sample by moving a stylus arm across the sample.
Thus stylus profilometers provide for scans in the X-Y plane for distances ranging from a few microns to hundreds of millimeters. The sensors used for profilometers usually have large dynamic range as well. For example, in stylus profilometers for sample height measurements, vertical variations in the Z direction as small as a few Angstroms to as large as a few hundred micrometers can be detected. Significantly, the height measurement profilometer measures height directly.
As the semiconductor industry has progressed to smaller dimensions with each new generation of products, there is an increasing need for scanning instruments that can repeatably scan samples to a very fine resolution. The large size of the X-Y stage in the stylus profilometer limits the lateral positioning resolution of the conventional stylus profilometer. Thus the repeatability of X-Y repositioning of stylus profilometers is limited to about 1 micrometer; such device lacks the capability for repeatable nanometer or sub-nanometer X-Y positioning capability.
It is therefore desirable to provide an improved scanning instrument that can provide better X-Y repeatable positioning resolution than the conventional stylus profilometer, while retaining many of the profilometer's advantages, such as wide dynamic range in the Z direction and long scan capability up to hundreds of millimeters.
It is desirable for semiconductor wafer surfaces to be flat or planar. To achieve such global planarization, Chemical Mechanical Polishing (CMP) is employed. CMP processing is typically applied after tungsten plugs, via holes have been fabricated on the surfaces of the semiconductor wafers. If the CMP processing is not functioning properly, it may cause a recess in the tungsten plug or via hole and, therefore, affect the size and depth of the tungsten plugs and via holes. This may lead to a variation of capacitance and electrical resistance across the surface of the semiconductor wafer which adversely affect the operation of electronic circuits fabricated on the wafer. The problem becomes particularly accute in vary large scale integration circuits where the size of transistors and other electronic devices have been continually reduced. This is true also for laser textured hard disks.
To monitor the functioning of CMP processing, scanning probe microscopes and profilometers have been used. While profilometers are able to provide a measure of the surface topography of the wafer, conventional profilometers lack the resolution to discover the shape and depth of the tungsten plugs or via holes, for example. Thus, if the profilometer scan did not pass over the tungsten plug or the via hole, information from the scan would not reveal such information. Conventional profilometers lack the position/positioning capability to allow precise alignment of submicron features with the scan. Hence, if profilometers are used for monitoring the CMP process, even though the global planarization of the sample and the relative height of points that are spaced apart on the wafer can be monitored, a precise local morphology of the surface cannot be measured.
While scanning probe microscopes (SPMs) do have the precision positioning capability to allow precise alignment of submicron features with the scan path, SPM devices do not have a precise long range and repeatable motion, so that it is difficult to use SPM devices to find out the relative locations of two points that are spaced far apart on the wafer surface or the height relationship between two tungsten plugs or via holes that are spaced apart on the wafer. As a matter of fact, in many SPM devices, any tilt experienced by the devices is considered as background and is subtracted. Even if a number of local images acquired by the SPM are stitched together, the global topography of the surface is lost, and height differences between points that are spaced that are spaced apart by distances beyond the range of SPM devices cannot be precisely measured. Moreover, data correlation between a number of local images of the SPM is cumbersome, time consuming and requires significant duplication of resources.
It is, therefore, desirable to provide an improved system which avoids the above-described difficulties.